The specific aim of the proposed research is to determine the detailed pattern of innervation of the sensory cells of the auditory organ in a variety of reptiles. Species chosen for study are some reptiles that show structural and functional characteristics similar to mammals, and other types that differ from mammals. The known range of auditory capability in the reptiles to be studied ranges from moderate to excellent. It is probable that differing innervational patterns will be related to functional capabilities. Since the reptilian auditory organ is the forerunner of the mammalian, detailed knowledge of its structure, and especially its innervation will be of great aid in understanding mammalian ear function. Study of the many different ways that reptiles have developed to subserve hearing are of importance in understanding basic auditory mechanisms. Preliminary studies of some reptilian species shows marked regional differences in the richness of sensory cell innervation in the auditory organ. These differences may be related to those found between the inner and outer hair cells of the mammalian organ of Corti. The primary method of study will be that of reconstructions of both serial thick (one micron) and thin (transmission electron microscopic) sections of the auditory epithelia. Determination of the number of efferent nerve fibers supplying the cochlea will be by section of the posterior root of the auditory nerve and subsequent study of degenerating fibers. Silver staining methods will be used to study the grosser distribution of nerve fibers to sensory cells. Injection of a dye (Lucifer Yellow) into the axons of the auditory nerve may aid in tracing the course of single neurons. Detailed information concerning the innervation of the auditory organ is fundamental to our understanding of hearing in man and other animals.